1. Field of the Invention
The present invention relates to a coil antenna used for a short distance communication system having an LF band (a long wave with frequencies of about 30 kHz to about 300 kHz).
2. Description of the Related Art
In a short distance communication system with an LF band (a long wave with frequencies of about 30 kHz to about 300 kHz), a coil antenna is made of a coil wound around a magnetic core (the coil antenna itself will be referred to as a winding structure below). The coil antenna is usually encased in a case.
FIG. 1 shows a configuration of a sending coil antenna 1 disclosed in Japanese Unexamined Patent Application Publication No. 2001-358522 (Patent Document 1). The coil antenna 1 includes a magnetic core 2, a bobbin 4 for accommodating the magnetic core 2, and a main coil 3 formed by winding a conducting wire around the bobbin 4. The coil antenna 1 also includes a case 5 accommodating the magnetic core 2, the bobbin 4, and the main coil 3 therein. Around the magnetic core 2, the bobbin 4, the main coil 3, and the case 5, a potting material is provided.
The magnetic core 2 includes a ferromagnetic substance, such as a ferromagnetic Mn—Zn ferrite, an amorphous magnetic substance other than the ferromagnetic Mn—Zn ferrite, and compaction molded magnetic impalpable powder. These magnetic substances have very low toughness and brittle breaking properties. When the toughness is further deteriorated due to the effects of temperature and humidity, the magnetic core 2 may fail when only a small load is applied thereto. Such failure of the magnetic core 2 may cause a change in resonance frequency, which destabilizes the radiant magnetic field of the coil antenna 1.
In Patent Document 1, the case 5 is fully packed with a potting material 10 by vacuum casting while bubbles generated in the potting material 10 are removed (such a conformation without bubbles will be referred to as a degasified component below). Thereby, the magnetic core 2 is prevented from being deteriorated due to temperature and humidity, and the magnetic core 2, the bobbin 4, and the main coil 3 are prevented from coming into contact with the case 5.
By making the degasified component 10 of a flexible rubber material, a static deformation and load applied to the case is absorbed due to the deformation of the degasified component 10, which prevents the magnetic core 2 from being applied to the static deformation and load via the degasified component 10.
However, since the case is packed with such a degasified component without leaving a space, when deformation is generated in or a load is applied to the case momentarily, the degasified component cannot deform (drift) and the responsiveness is not so good. Hence, the deformation and the load are momentarily transmitted to the magnetic core, which leads to damage of the magnetic core.
When the case is filled with the degasified component by the vacuum casting, displacement is generated in the magnetic core due to the deformation of the degasified component during curing, which may cause damage to the magnetic core due to a thin portion of the degasified component or the hardening of the magnetic core having an external force applied thereto.